Method for manufacturing semiconductor substrate

ABSTRACT

An epitaxial layer is formed on a high-resistance semiconductor substrate containing interstitial oxygen at a high concentration, and then a heat treatment is performed to the semiconductor substrate at a high temperature in an oxidizing atmosphere. Accordingly, a stratiform region of SiO 2  is formed by deposition at an interface between the epitaxial layer and the semiconductor substrate. As a result, an apparent SOI substrate for an SOI semiconductor device can be manufactured at a low cost.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/713,018 field Nov. 16, 2000 now U.S. Pat. No. 6,676,748,which is based upon and claims benefit of Japanese Patent ApplicationsNo. 11-326934 filed on Nov. 17, 1999, and No. 2000-333286 filed on Oct.31, 2000, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for manufacturing a semiconductorsubstrate that is generally used for a semiconductor device including acomposite IC and an LSI.

2. Description of the Related Art

An SOI (Silicon On Insulator) semiconductor device has a semiconductorlayer that is disposed on a semiconductor substrate through anintermediate insulating layer. Such an SOI substrate is suitably usedfor a device such as a composite IC, a high withstand voltage IC or anLSI for a portable instrument that is required to have high speed andlow consumption power, in which several kinds of elements such asbipolar, MOS, and power elements are mounted on one chip.

To manufacture the SOI semiconductor device, an SOI substrate isrequired, which includes a high-quality crystalline semiconductor layerthat is formed on a layer made of an insulating material such as SiO₂with extremely high resistance. Known conventional methods formanufacturing the SOI substrates include a bonding method, a SIMOXmethod, a method that combines bonding and ion implantation by utilizinghydrogen brittleness, and the like.

However, the SOI substrate manufactured by conventional techniques inany of the above-described methods is several to several dozen timesmore expensive than an ordinary bulk substrate. This is the biggestreason for preventing the SOI semiconductor device from beingpractically used, regardless of its inherent high performance and highfunctionality.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems. Anobject of the present invention is to provide a method for manufacturinga semiconductor substrate suitably used for an SOI semiconductor device,with high quality at low cost.

Briefly, according to a first aspect of the present invention, after anepitaxial layer is formed on a semiconductor substrate, an insulatinglayer is formed by deposition at an interface between the epitaxiallayer and the semiconductor substrate by performing a heat treatment inan oxidizing atmosphere. Thus, the semiconductor substrate for an SOIsemiconductor device can be manufactured easily at low cost. A thicknessof an SOI layer required for the semiconductor device can be determinedby the thickness of the epitaxial layer.

According to a second aspect of the present invention, an apparent SOIsubstrate can be formed by epitaxially growing a semiconductor layer ona semi-insulating substrate having a high resistance. Preferably, beforethe semiconductor layer is epitaxially grown on the substrate, a heattreatment is performed in a hydrogen atmosphere to improve crystallinityon a surface of the semiconductor substrate. Accordingly, thecrystallinity of the semiconductor layer is further improved.

According to a third aspect of the present invention, a base wafer and abonding wafer are prepared, one of which is composed of a semiconductorsubstrate containing oxygen at a high concentration or a semi-insulatingsemiconductor substrate having a high resistance. An oxide film isformed on one of the base wafer and the bonding wafer. Then, the basewafer and the bonding wafer are bonded together with the oxide filminterposed therebetween. After that, a back surface of the bonding waferat an opposite side of the base wafer is ground and polished to form anSOI layer on the base wafer through the oxide film.

According to fourth aspect of the present invention, first, an elementis ion-implanted into a high resistance semiconductor substrate,containing oxygen at a high concentration, to form a deposition nuclearlayer by the element. The deposition nuclear layer has a plurality ofnuclei for deposition and extends at a depth from a surface of thesemiconductor substrate. Then, a heat treatment is performed to thesemiconductor substrate to form an oxide layer in the semiconductorsubstrate by making the oxygen, contained in the semiconductorsubstrate, deposit using the plurality of nuclei in the depositionnuclear layer.

According to the present invention described above, in any case, thesemiconductor substrate for an SOI semiconductor device can bemanufactured with high quality at significantly reduced low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become morereadily apparent from a better understanding of the preferredembodiments described below with reference to the following drawings, inwhich;

FIGS. 1A and 1B are cross-sectional views showing steps formanufacturing a semiconductor substrate for an SOI semiconductor devicein a first preferred embodiment;

FIG. 2 is a cross-sectional view showing a step for manufacturing asemiconductor substrate for an SOI semiconductor device in a secondpreferred embodiment;

FIG. 3 is a graph showing a relation between hFE of a parasitictransistor and minority carrier lifetime;

FIG. 4 is a cross-sectional view showing a step for manufacturing asemiconductor substrate for an SOI semiconductor device in a thirdpreferred embodiment;

FIGS. 5A to 5C are cross-sectional views showing steps for manufacturinga semiconductor substrate for an SOI semiconductor device in a fourthpreferred embodiment;

FIGS. 6A to 6D are cross-sectional views showing steps for manufacturinga semiconductor substrate for an SOI semiconductor device in a fifthpreferred embodiment;

FIGS. 7A to 7C are cross-sectional views showing steps for manufacturinga semiconductor substrate for an SOI semiconductor device in a sixthpreferred embodiment;

FIGS. 8A to 8D are cross-sectional views showing steps for manufacturinga semiconductor substrate for an SOI semiconductor device in a seventhpreferred embodiment;

FIGS. 9A to 9D are cross-sectional views showing steps for manufacturinga semiconductor substrate for an SOI semiconductor device in an eighthpreferred embodiment;

FIGS. 10A to 10D are cross-sectional views showing steps formanufacturing a semiconductor substrate for an SOI semiconductor devicein a ninth preferred embodiment;

FIGS. 11A to 11E are cross-sectional views showing steps formanufacturing a semiconductor substrate for an SOI semiconductor devicein a tenth preferred embodiment;

FIG. 12 is a cross-sectional view showing a device adopting the SOIsubstrate manufactured by the steps shown in FIGS. 11A to 11E; and

FIGS. 13A to 13E are cross-sectional views showing various insulatingisolation structures to which the substrates manufactured in theembodiments of the present invention can be applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

A method for manufacturing a semiconductor substrate for an SOIsemiconductor device to which a first preferred embodiment of theinvention is applied is explained referring to FIGS. 1A and 1B.

In the first embodiment, a high-resistance semiconductor substrate 1having a mirror-finished surface and including interstitial oxygen at ahigh concentration is used as a substrate for epitaxial growth. Whilesilicon single crystal that has been grown by a CZ method containsoxygen of about 10¹⁷ atoms/cm³ among lattices therein, a mirror wafercontaining interstitial oxygen at a higher concentration of, forexample, more than 1×10¹⁸ atoms/cm³ is used as a start material in thisembodiment. The mirror wafer can be manufactured by the CZ methodsimilarly to ordinary mirror wafers.

This semiconductor substrate 1 undergoes a pre-cleaning treatmentincluding, for example, an immersion treatment into SC-1 solution(mixture composed of NH₄OH, H₂O₂, and H₂O, APM solution), an immersiontreatment into SC-2 solution (mixture composed of HCl, H₂O₂, and H₂O,HPM solution), an immersion treatment into dilute HF solution,super-pure water substitution, and drying. Then, an epitaxial layer(semiconductor active layer) 2 is formed in accordance with a requiredthickness by epitaxial growth involving HCl etching, H₂ gassubstitution, and the like within an epitaxial apparatus.

After that, the semiconductor substrate 1 on which the epitaxial layer 2is formed is heated at a high temperature of, for example, 1150° C. ormore, in oxidizing atmosphere. Accordingly, oxygen in thehigh-resistance semiconductor substrate 1 containing interstitial oxygenat a high concentration is deposited using as nuclei a distortion layerat the interface between the epitaxial layer 2 and the semiconductorsubstrate 1. In consequence, a stratiform region (oxide film) 3 of SiO₂is formed at the interface between the epitaxial layer 2 and thesemiconductor substrate 1, thereby forming an SOI structure. Thestratiform region 3 of SiO₂ formed at the interface is about 100 nm inthickness. However, since the semiconductor substrate 1 used has highresistance, the stratiform region 3 can electrically isolate elements incooperation with trench isolation, and realize performances equivalentto those of an ordinary SOI substrate.

The method for manufacturing the semiconductor substrate for an SOIsemiconductor device described above can dispense with many steps suchas preparation of two mirror wafers for bonding, bonding of the twomirror wafers, heat treatment for bonding, edge treatment for obtaininga required SOI thickness, surface grinding, re-polishing for mirrorfinish, and several checks for voids, SOI thickness, and the like, incomparison with a conventional bonding method. In consequence,significant cost reduction can be achieved. Also, in comparison with aSIMOX method, the SIMOX method necessitates an expensive apparatus andits throughput is low because oxygen must be ion-implanted into asemiconductor substrate with high energy to have a high concentration(1×10¹⁸ cm⁻³). To the contrary, in the present invention, the SiO₂stratiform region 3 can be formed by using the semiconductor substratecontaining interstitial oxygen at a high concentration, and performingonly the ordinary epitaxial growth for the active layer and the heattreatment in oxidizing atmosphere. Therefore, significant cost reductioncan be realized in the present embodiment.

(Second Embodiment)

FIG. 2 shows a second preferred embodiment of the present invention. Inthis embodiment, a semi-insulating semiconductor substrate 11 is used inplace of the semiconductor substrate 1 used in the first embodiment. Anepitaxial layer 12 can be formed on the semiconductor substrate 11similarly to the first embodiment. Accordingly, an apparent SOIstructure can be constructed without performing an oxygen depositionheat treatment at a high temperature in oxidizing atmosphere after theepitaxial layer 12 is formed.

For example, a substrate having a lifetime of a minority carrier(minority carrier lifetime) less than about 1×10⁻⁸ sec and a carrierconcentration less than about 1×10¹⁴ cm⁻³ can be used as thesemi-insulating semiconductor substrate 11 in this embodiment. This isbecause, in a state where elements are formed with impurity layers inthe epitaxial layer 12 and the semiconductor substrate 11 adjacently toeach other, hFE of a parasitic transistor formed by the impurity layersof the adjacent elements and the semiconductor substrate 11 and theminority carrier lifetime τ g have a relation as shown in FIG. 3. Thatis, referring to FIG. 3, it is preferable that hFE of the parasitictransistor is less than about 10⁻¹ to negligibly decrease the effect bythe parasitic transistor, and hFE of the parasitic transistor becomesless than about 10⁻¹ when the minority carrier lifetime τ g is less thanabut 1×10⁻⁸ sec. Therefore, the minority carrier lifetime is determinedas described above.

The carrier concentration of the semiconductor substrate 11 is notlimited, and for example, may be 1×10¹⁴ cm⁻³ or less. Thesemi-insulating semiconductor substrate 11 may be a substrate containingan impurity that forms a deep trap level in a bandgap of highconcentration interstitial oxygen, carbon, or the like.

(Third Embodiment)

FIG. 4 shows a third preferred embodiment of the present invention. Inthis embodiment, a semi-insulating semiconductor substrate 21 doped witha dopant, a conductivity type of which is opposite to that of anepitaxial layer 22 is used in place of the semiconductor substrate 11 inthe second embodiment. As shown in the figure, specifically, when theepitaxial layer 22 is n type, the semiconductor substrate 21 is p type,and when the epitaxial layer 22 is p type, the semiconductor substrate21 is n type. Accordingly, because a PN junction is provided between theepitaxial layer 22 and the semiconductor substrate 21, electricalinsulation can be achieved more securely than in the second embodiment.

(Fourth Embodiment)

FIGS. 5A to 5C show a fourth preferred embodiment of the presentinvention. In this embodiment, similarly to the first to thirdembodiments, a high-resistance semiconductor substrate containinginterstitial oxygen at a high concentration or semi-insulating substrateis used as a semiconductor substrate 31 (FIG. 5A). Then, a hightemperature heat treatment is performed to the semiconductor substrate31 at, for example, 1000° C. or more in hydrogen atmosphere beforeperforming epitaxial growth.

Accordingly, there arise both or either of phenomena that interstitialoxygen atoms contained in the substrate are outwardly diffused andreleased from the surface of the substrate, and that atoms forming thesurface of the substrate are rearranged. Then, a layer 32 is formed atthe substrate surface by outward diffusion of oxygen or/andrearrangement of atoms (FIG. 5B), so that the crystallinity of thesubstrate surface is improved. Because of this, an epitaxial layer 33formed thereafter can have further improved crystallinity.

(Fifth Embodiment)

FIGS. 6A to 6D show a fifth preferred embodiment of the presentinvention. Incidentally, steps shown in FIGS. 6A to 6C are substantiallythe same as those shown in FIGS. 5A to 5C. In this embodiment, anepitaxial growth substrate that is fabricated by the method described inthe fourth embodiment is heated at a high temperature of, for example,1150° C. or more, in oxidizing atmosphere (FIG. 6D).

Accordingly, oxygen in the high-resistance semiconductor substrate 31that contains interstitial oxygen at a high concentration is depositedusing as nuclei a distortion layer at the interface between theepitaxial growth layer and the substrate so that a SiO₂ stratiformregion 34 can be formed by additionally performing the heat treatment inthe oxidizing atmosphere as in the first embodiment.

(Sixth Embodiment)

FIGS. 7A to 7C show a sixth preferred embodiment of the presentinvention. In this embodiment, as in the first to third embodiments, ahigh-resistance semiconductor substrate containing interstitial oxygenat a high concentration or semi-insulating substrate is used as asemiconductor substrate 41 (FIG. 7A), and a thin semiconductor layer 42is epitaxially grown to have a conductive type opposite to that of anepitaxial layer 43 that is formed in a subsequent step as an activelayer (FIG. 7B). Successively, the epitaxial layer 43 is epitaxial grown(FIG. 7C). For example, when the active layer (epitaxial layer 43) isformed to be an n⁻ type layer, the semiconductor layer 42 is formed tobe a p⁻ type layer.

According to this manufacturing method, the semiconductor layer 42,which is formed at the interface between the semiconductor substrate 41and the epitaxial layer 43 and has the conductivity type opposite tothat of the active layer, can be completely depleted to support voltageand to perform insulating isolation in cooperation with the underlyinghigh-resistance semiconductor substrate 41. As a result, thesemiconductor layer 42 can provide an apparent SOI structure. As in thefirst embodiment, a heat treatment may be performed in high-temperatureoxidizing atmosphere to form an oxide layer deposited.

(Seventh Embodiment)

FIGS. 8A to 8D show a seventh preferred embodiment of the presentinvention. In this embodiment, before the epitaxial growth in the sixthembodiment is performed, similarly to the fourth and fifth embodiments,the step shown in FIG. 5B is performed to the semiconductor substrate41. That is, a high temperature heat treatment is performed at, forexample, 1000° C. or more in hydrogen atmosphere. Accordingly, both oreither of phenomenon that interstitial oxygen contained in thesemiconductor substrate 41 is outwardly diffused to be released from thesubstrate surface, and atoms constituting the substrate surface arerearranged occur, and a layer 44 is formed at the substrate surface dueto outward diffusion of oxygen and rearrangement of atoms (FIG. 8B). Asa result, the crystallinity of the substrate surface is improved.

After that, as shown in FIGS. 8C and 8D, the similar steps to thoseshown in FIGS. 7B and 7C are performed to form an apparent SOIsubstrate. Incidentally, also in the present embodiment, a heattreatment may be performed in a high-temperature oxidizing atmosphere tomake an oxide layer deposited as in the fifth embodiment.

(Eighth Embodiment)

FIGS. 9A to 9D shows an eighth preferred embodiment of the presentinvention. This embodiment uses a high-resistance semiconductorsubstrate 500 having a mirror-finished surface and containing oxygen ata high concentration (FIG. 9A). Then, first, a pad oxide film (notshown) having a thickness of about 45 nm is formed on the surface byperforming a heat treatment in oxidizing atmosphere. This step isperformed in an ordinary semiconductor process to prevent occurrence ofchanneling components along crystal axes and sputters on the surface byion implantation.

Next, for example, oxygen ions are implanted into the substrate 500through the pad oxide film at about 1×10¹⁶ cm⁻² (FIG. 9B). Anacceleration voltage in this case was 100 to 180 KeV in this embodiment,which was determined in accordance with a depth of implantation.Accordingly, nuclei for depositing dissolved oxygen in the substrate 500can be formed as a deposition nuclear layer 501 shown in FIG. 9B.Implantation of oxygen ions for forming an SOI substrate is known in theSIMOX method; however, in the case of the SIMOX method, a dose isgenerally about 1×10¹⁸ cm⁻², which is larger than that of the presentembodiment by two digits.

After that, a heat treatment is performed to the semiconductor substrate500 at a temperature of, for example, 1100° C. or more in nitrogen oroxygen atmosphere for 18 to 35 hours (FIG. 9C). Accordingly, dissolvedoxygen in the semiconductor substrate 500 is deposited using implantedoxygen ions as nuclei in the layer 501 so that an oxide layer, i.e., aSiOx layer 502 is formed as shown in FIG. 9C, thereby forming an SOIsubstrate 504. Here, a value of x was about 2 at the heat treatmentconditions described above.

In this embodiment, because the substrate 500 is composed of ahigh-resistance semiconductor substrate, a high-resistance semiconductorlayer underlies the deposited oxide layer 502, and a depletion layer isformed when a voltage is applied across the oxide layer. As a result, alarger withstand voltage than that determined by the thickness of theoxide film can be exhibited. In this embodiment, although oxygen ision-implanted as an element for forming deposition nuclei, otherelements such as nitrogen, silicon, carbon, and fluorine can be used inplace of oxygen, which are liable to combine with oxygen to bedeposited.

As shown in FIG. 9D, when a semiconductor layer 503 having apredetermined conductive type and a thickness is epitaxially grown onthe SOI substrate 504 manufactured as above, an SOI substrate can beformed with desirable film thickness, conductive type, andconcentration.

(Ninth Embodiment)

FIGS. 10A to 10D show a ninth preferred embodiment of the presentinvention. In this embodiment, a high-resistance semiconductor substratecontaining interstitial oxygen at a high concentration orsemi-insulating substrate as disclosed in the first to three embodimentsis prepared as a base wafer 51, and an ordinary mirror wafer is preparedas a bonding wafer 52 (FIG. 10A). Then, an oxide film 53 is formed on amirror-finished principal surface of at least one of the base wafer 51and the bonding wafer 52 (FIG. 10B), and the two wafers are bondedtogether at the principal surfaces thereof in clean atmosphere by anordinary wafer bonding method, and a high-temperature heat treatment isperformed to thereby form a combined wafer 54 (FIG. 10C). After that,the back surface of the combined wafer 54 at the side of the bondingwafer 52 is ground and polished for mirror finishing so that an SOIlayer have a predetermined thickness. As a result, an SOI substrate ismanufactured (FIG. 10D).

In this embodiment, unlike a conventional manufacturing method, becausethe high-resistance semiconductor substrate containing interstitialoxygen at a high concentration or semi-insulating substrate is used asthe base wafer, an SOI substrate having a high withstand voltage of, forexample, 200 V or more can be attained with a thin embedded oxide filmthickness of about several hundreds nm that is about 1/10 thinner thanthat of a conventional one.

(Tenth Embodiment)

FIGS. 11A to 11E shows a tenth preferred embodiment of the presentinvention. In this embodiment, a high-resistance semiconductor substratecontaining interstitial oxygen at a high concentration orsemi-insulating substrate as described in the first to third embodimentsis prepared as a bonding wafer 61, while an ordinary mirror wafer isprepared as a base wafer 62 (FIG. 11A). Then, an oxide film 63 is formedon a mirror-finished principal surface of at least one of the bondingwafer 61 and the base wafer 62 (FIG. 11B), and the two wafers are bondedtogether at the principal surfaces thereof in clean atmosphere by anordinary wafer bonding method, and a high-temperature heat treatment isperformed to thereby form a combined wafer 64 (FIG. 11C).

After that, the back surface of the combined wafer 64 at the side of thebonding wafer 61 is ground and polished for mirror finishing. As aresult, an SOI substrate is manufactured with an SOI layer having arequired thickness (FIG. 11D). Further, oxygen on the SOI layer surfaceis outwardly diffused by a heat treatment performed at a hightemperature in hydrogen atmosphere. Accordingly, oxygen remains at thebonding interface, and gettering sites are formed at that portion (FIG.11E). The gettering sites formed in the SOI layer can take heavy metalcontaminants in when an oxide film is formed on the SOI layer, andtherefore lengthen the lifetime of the oxide film.

For example, the SOI substrate manufactured as described in thisembodiment can be used for a device shown in FIG. 12. This device isformed with an LDMOS 70, a bipolar transistor 80, a CMOS 90, and a diode100.

The LDMOS 70 is composed of a p type base region 71 formed at a surfaceportion of the n⁻ type SOI layer (bonding wafer 61), an n⁺ type sourceregion 72 formed in a surface portion of the p type base region 71, ann⁺ type drain region 73 formed in a surface portion of the SOI layerremotely from the p type base region 71, a gate insulating film 74formed at least on the p type base region 71, a gate electrode 75 formedon the gate insulating film 74, a source electrode 76 electricallyconnected to the n⁺ type source region 72, and a drain electrode 77electrically connected to the n⁺ type drain region 73.

The bipolar transistor 80 is composed of a p type base region 81 formedon a surface portion of the SOI layer, an n⁺ type emitter region 82formed in a surface portion of the p type base region 81, an n⁺ typecollector region 83 formed in a surface portion of the SOI layerremotely from the p type base region 81, and a base electrode 84, anemitter electrode 85, and a collector electrode 86 electricallyconnected to these regions, respectively.

The CMOS 90 is composed of an n type well layer 91 and a p type welllayer 92, which are formed in a surface portion of the SOI layer, p⁺type source 93 a and drain 94 a formed in the n type well layer 91separately from each other, n⁺ type source 93 b and drain 94 b formed inthe p type well layer 92 separately from each other, gate insulatingfilms 95 a, 95 b and gate electrodes 96 a, 96 b respectively providedabove channel regions between the respective sources 93 a, 93 b and therespective drains 94 a, 94 b, source electrodes 97 a, 97 b respectivelyconnected to the sources 93 a, 93 b, and drain electrodes 98 a, 98 brespectively connected to the drains 94 a, 94 b.

The diode 100 is composed of a p type region 101 and a p⁺ type contactregion 102 formed in a surface portion of the SOI layer, an n⁺ typeregion 103 provided remotely from the p type region 101, and anode andcathode electrodes 104, 105 electrically connected to the respectiveregions 101, 103.

In this device, because gettering sites are formed in the. SOI layer ofthe SOI substrate manufactured in this embodiment, the following effectscan be attained when the SOI substrate is used for the LDMOS 70, theCMOS 90 and the diode 100.

Specifically, in the case of elements such as the LDMOS 70 and the CMOS90 having the gate insulating films 74, 95 a, 95 b, because thegettering sites take heavy metal contaminants in, the gate insulatingfilms 74, 95 a, 95 b can be improved in lifetime. This results inimproved reliability of the elements.

Besides, in the case of the CMOS 90 in which both the n type well layer91 and the p type well layer 92 are formed, it is preferable to isolatethe layers from each other by a trench in consideration of latch upprevention. However, there is a case where the trench isolation is notprovided to reduce the size of the device. Even in such a case, thegettering sites can prevent latch up.

Further, when an operational state is switched from ON to OFF in thediode 100, holes injected into the n⁻ type SOI layer from the anodeelectrode return into the anode electrode to generate current flow in aninverse direction. However, if gettering sites exist, the getteringsites trap holes as trap sites, and make the holes recombine withelectrons. As a result, the holes disappear apparently, and no currentflows in the inverse direction. The diode 100 can be improved inrecovery property. Incidentally, though it is not shown in FIG. 12,since an IGBT can have current flow in an inverse direction similarly tothe diode 100, the SOI substrate shown in this embodiment can be usedfor formation of the IGBT to improve the recovery property of the IGBT.

The above-described embodiments exemplify oxygen arranged among latticesother than lattice points; however, oxygen may be arranged at otherpositions to provide the same effects as described above. Especially,oxygen contained in the semiconductor substrates 1, 21, 31, 41, and 51may not be interstitial oxygen. Also, in the above-describedembodiments, the semiconductor substrates 1, 21, 31, 41, and 51 arerespectively composed of high-resistance substrates; however, thesubstrates can provide the same effects as described above even whenthey do not have high resistance.

In the first and fifth embodiments, although it is explained that theoxide film is deposited by the heat treatment performed in oxidizingatmosphere, it is possible to deposit other insulting layers. Forexample, a nitride layer can be deposited using as nuclei partiallyexisting nitrogen in a substrate or the like. Thus, the insulating layercan form an apparent SOI substrate. In this case, the semiconductorsubstrate has no need to contain oxygen therein.

Incidentally, various insulating isolation structures can be formed bythe substrates as manufactured in the above-described embodiments.Examples are shown in FIGS. 13A to 13E, in which a substrate having a PNjunction as described in the third embodiment is used, but the othersubstrates in the other embodiments can also be used as well.

For example, as shown in FIG. 13A, a well-isolation structure is formedby forming a well layer 110 in the n⁻ type epitaxial layer with aninverse conductive type to that of the epitaxial layer 22 to contact thesemi-insulating substrate 21. Otherwise, as shown in FIG. 13B, a trenchisolation structure can be formed by forming a trench 111 in theepitaxial layer 22 so that the trench reaches the semi-insulatingsubstrate 21, and by filling the trench 111 with an insulating film 112.Otherwise, as shown in FIG. 13C, a well-trench isolation structure canbe formed by combining the structures shown in FIGS. 13A and 13B. Asshown in FIG. 13D, a double-trench isolation structure may be formed, inwhich two trenches each of which is similar to that shown in FIG. 13Bare formed adjacently to each other. FIG. 13E shows a double-trenchisolation structure in which a region interposed between two trenches ismade a well layer 113 having the same conductivity type as that of thesemi-insulating substrate 21, and the well layer 113 is grounded forparasitic removal.

While the present invention has been shown and described with referenceto the foregoing preferred embodiments, it will be apparent to thoseskilled in the art that changes in form and detail may be made thereinwithout departing from the scope of the invention as defined in theappended claims.

1. A method for manufacturing a semiconductor substrate, comprising:forming an epitaxial layer on a semiconductor substrate by epitaxialgrowth; and forming an insulating layer by deposition at an interfacebetween the epitaxial layer and the semiconductor substrate byperforming a heat treatment that is performed in an oxidizingatmosphere, the insulating layer is formed in a such manner that theinsulating layer has a thickness of about one hundred nanometers and astratiform region of the insulating layer at the interface between theepitaxial layer and the semiconductor substrate is provided, wherein:the heat treatment for forming the insulating layer is performed at atemperature higher than about 1100° C.; the insulating layer is formedfrom a distortion layer as nuclei disposed at the interface between theepitaxial layer and the semiconductor substrate; and the distortionlayer is formed by a difference of impurity concentration or a latticeconstant between the epitaxial layer and the semiconductor substrate. 2.The method according to claim 1, wherein: the semiconductor substratecontains oxygen at a high concentration; and the insulating layer isformed by deposition of an oxide.
 3. The method according to claim 2,wherein the concentration of oxygen in the semiconductor substrate ismore than about 1×10¹⁸ atoms/cm³.
 4. The method according to claim 1,wherein: the semiconductor substrate is composed of one of a substratecontaining oxygen at a high concentration and a semi-insulatingsubstrate having a high resistance; and a crystallinity improving heattreatment is performed to the semiconductor substrate in a hydrogenatmosphere to improve crystallinity on a surface of the semiconductorsubstrate before the epitaxial layer is formed on the surface of thesemiconductor substrate.
 5. The method according to claim 1, wherein thesemiconductor substrate is made of silicon, the epixtaxial layer is madeof silicon, and the insulating layer is made of silicon oxide.